CDIP has used a wide range of instrumentation over the years. Because
data has been collected from such a diverse assortment of sites - from
inside harbors and basins, from the pilings of piers and oil platforms,
from isolated nearshore and remote offshore locations - each sensor has
to be carefully chosen and prepared for its specific environment. In
addition to enviromental factors, technological advances have also
spurred changes in CDIP’s instrumentation. Over the decades the program
has been in operation, sensors have become much more refined and
accurate.

The focus of CDIP’s instrument development and use has naturally been on
wave measurement. Two main approaches have been pursued - using fixed
underwater sensors, which measure the height of waves passing above, and
using surface buoys, which record their own motions with the sea
surface.

Single point gauges are pressure sensors which are mounted at a fixed
position underwater. These instruments measure pressure fluctuations -
the changing height of the water column above the sensor - associated
with passing waves. These pressure time series can be converted to sea
surface elevations and wave frequency spectra. CDIP’s very first
deployment, at Imperial Beach in 1975, was of a single-point gauge. A
single-point gauge is presently still in use at Scripps Pier.

Single point tripod, San Francisco, 1998

Single point gauges are generally used close to shore, where they can
be cabled directly to a land-based field station that records the
data. In areas where there are existing structures like piers, the
single-point gauges can be mounted directly against the sub-surface
portions of pilings or other features. In other areas, the gauges are
deployed atop tripod mounts.

CDIP has used pressure sensors from a number of manufacturers, such as
Paroscientific and Sensotec. For specifications and more information on
the sensors themselves, check the links below.

Although single-point gauges can effectively measure wave height and
period, they can not be used to determine the direction waves are
travelling. When multiple underwater pressure sensors are placed in
close proximity, however, comparison of their measurements does allow
direction to be resolved. To take advantage of this fact, CDIP developed
slope arrays which allow directional wave analysis. The first array was
deployed at Santa Cruz in 1977, and the most recent array, at Kings Bay,
Georgia was in service from 1995 to 2006.

CDIP’s standard array formation consists of four pressure sensors placed
in a six-by-six meter square. Like the single-point gauges, arrays are
generally used in nearshore waters and cabled directly to shore. In a
few instances, however, CDIP has deployed arrays on offshore platforms,
e.g. on Harvest Platform in California and at Kings Bay, Georgia. In
these cases, the geometry of the array varies with the platform
configuration, and the data is recorded on the platform and sent via
radio link back to shore.

Given the dynamic nature of the ocean environment, deploying and
maintaining wave sensors can be a very demanding undertaking. As CDIP
has shifted emphasis from pressure sensors to buoys, the nature of this
work has changed, but always remained challenging.

A helicopter deployment

Up through the early 1990s, when CDIP was primarily using pressure
sensors and arrays, much of the challenge stemmed from the fact that
the instruments were being deployed close to the surf zone and had to be
connected by cable to a shore station. While installations on
existing structures such as piers or harbor pilings were relatively
straightforward, other installations - especially on high-energy beaches
- could be very involved. In some cases, CDIP had to rely on
amphibious vehicles (LARC) or helicopters to transport instruments
outside the surf zone, lower them to the bottom, and lay cable back to
shore.

When measuring waves far offshore or at remote locations, buoys are
often a more practical solution than pressure sensors. Riding on the
ocean surface, buoys do not need to be linked by a cable to shore; with
an antenna on top of the buoy, data can be transmitted via a radio link.
They are also easier to deploy, move, and recover, securely attached to
a mooring that anchors them to the bottom.

The first buoys used by CDIP were non-directional, measuring wave
energy only. Equipped with one accelerometer, these instruments
measure wave height by recording the vertical acceleration of the
buoy as it rises and falls with passing waves. Over the years from 1978 to 1998,
two types of non-directional buoys were used - Wavecrest buoys and Waverider
buoys. The Waverider, a 0.9-meter spherical buoy produced by Datawell b.v.,
was by far the most frequently used by CDIP.

A Datawell Waverider buoy

Buoys follow the movement of the sea surface quite effectively. While a
buoy rises and falls with the waves the force of the mooring line does
in fact change, but this has only a small effect on the buoy’s response.
The Waverider, for instance, can effectively track waves with periods
down to 1.6 seconds, recording vertical displacements with a maximum
error of 3%. (Below 1.6 seconds, however, the Waverider’s response
quickly decreases.)

During the 1990s, CDIP gradually replaced all of its non-directional
buoys with directional buoys manufactured by Datawell. These buoys use
hippy heave-pitch-roll sensors to measure wave direction as well as wave
energy. They also have a pair of horizontal accelerometers for measuring
north/south and east/west displacements. This translational system uses
pitch-roll to correct the buoy movement to a fixed x-y-z reference
frame; it has a better signal-to-noise ratio than buoys that use
pitch-roll directly to estimate directional wave properties.

Datawell Directional Waverider buoy (Mark III)

Datawell’s Directional Waverider buoy is now the primary instrument
deployed and operated by CDIP. Like the non-directional Waverider, it
effectively measures waves with periods from 1.6 to 30 seconds, with
an error of no more than 3%. It also measures wave direction and sea
surface temperature. The temperature sensor is located at the base of
the .9 meter spehrical buoy, approximately 45 cm or 18 inches below
the surface. In accordance with IALA standards, the buoys are equipped
with a light that flashes 5 times in a 20-second cycle (5 flashes 2
seconds apart, followed by a 12-second pause).

Datawell Directional Waverider 4

As of 2017, CDIP is now phasing in Datawell’s waverider 4 buoy,
a significant improvement over the previous model.
It now samples the accelerometer at twice the rate of the
Mark III buoy thereby measuring wave height with wave periods of 1 to 30 seconds
with an accuracy of 0.5% of measured value. In addition to
measuring wave direction and sea surface temperature, it can
also measure surface current using 3 acoustic transducers
integrated into the hull. The messaging protocol is completely re-worked
to achieve the required flexibility to send different measured data types
with different sample rates. Couple this with the solar cell and Cunifer10
hull options, this sensor is easy to handle and deploy, low maintenance and very robust.

CDIP constructed a Calibration Facility based on Datawell’s own facility
design in order to calibrate its directional wave buoys before
deployment. The facility allows CDIP to test buoys with variable
frequencies which imitate the ocean’s different wave conditions.

Buoys present a different set of challenges. Although they do not need
to be cabled to shore, they do need a secure anchor and mooring line. In
most cases, the 400-lb directional buoys are anchored with about 1200
pounds of ballast chain. This anchor is attached to a mooring line
approximately twice the water depth at the buoys location. For a buoy
like those at Point Conception or Point Reyes, in over 500 meters of
water, a mooring system of over 1000m may be used. Laying out all of
this mooring and then dropping the anchor and buoy at precisely defined
coordinates in the open ocean is not an easy task. In Southern
California waters, CDIP uses its own work boat, a 26’ Wilson named the
SeaDipper, for deployments and recoveries. Further afield, the Coast
Guard and other organizations often lend assistance.

Once the instruments have been deployed, service and maintenance
needs are generally minimal. For single-point sensors and arrays,
uninterrupted data acquisition commonly proceeds for years at a time.
There is no limit to their life, since they are powered through the
cable that attaches them to shore. Buoys, on the other hand, require
more attention since they are powered by batteries in the buoy hull.
Approximately every 2 to 3 years, buoys are swapped out and replaced
with newly-calibrated buoys filled with fresh batteries. Despite the
longevity of the sensors, CDIP staffers enjoy getting in the water,
and periodically dive to work on instruments and their moorings.

Although CDIP focuses on the collection of wave data, at some
sites wind, pressure, and temperature data are collected to
complement our wave measurements. In addition to sea surface temperatures
for directional buoys, CDIP has collected mid-column, sea bottom,
and air temperatures at Scripps Pier. At Kings Bay, Georgia, air
pressure measurements have been made. And winds - so important for
the generation of waves - have been measured at a number of sites.

The CDIP wind buoy

For the most part, CDIP’s wind, temperature, and air pressure
measurements have been made with standard instrumentation. One
exception to this is the CDIP wind buoy, an instrument specially
designed for deployment in proximity to a wave buoy. Built from the hull
of a Waverider buoy, the wind buoy is powered by solar panels and
batteries, and collects wind data with an R.M. Young
anemometer positioned 2.8 meters above the sea surface. Wind
direction is calculated by comparing the direction recorded by the
anemometer with that of a digital compass in the hull of the buoy.